Presently, implants are usually machined from materials in bulk form, obtained for example from extruded rods or sheets. They may consist of polyethylene (UHMWPe), ceramic or metal alloys such as cobalt-chrome or stainless steel. Material waste is therefore high and may result in excessive costs.
The surfaces of bearings in hip, knee and shoulder implants generally fall into one of the following categories: (1) polyethylene on metal, (2) polyethylene on ceramic, (3) ceramic on ceramic, (4) ceramic on metal, or (5) metal on metal bearings.
Ceramic has the advantage of a very low friction coefficient. It is, however, heavy, relatively expensive and prone to fracture upon impact. In addition, geometric restrictions exist for this family of materials.
Metal alloys, when used in conjunction with polyethylene or ceramic, do function well, but concerns exist about polyethylene wear with a metal alloy counterpart due to the relatively high friction coefficient of this pairing.
Metal on metal bearings are currently causing great concern due to feared release of metal ions. Friction coefficients are relatively high and therefore require post-machining to a level of “super-finishing”.
The greatest long-term impact on implant survival is, however, due to polyethylene wear. The literature seems to suggest that the biggest cause of long-term implant loosening and ultimately implant failure is polyethylene wear. When polyethylene debris is released into the joint capsule either due to friction wear or monomer particle release, giant cells called macrophages form around the debris, as the body is not capable of absorbing polyethylene debris. As more macrophages form, the implant can become unstable and fail.
Polyaryl-ether-ether-ketone aka Polyether Ether Ketone Compound (PEEK) is a material renowned for its stability and strength (see for example S. M. Kurtz et al., Biomaterials 28 (32), pp. 4845-4869 (2007), the entire disclosure of which is hereby incorporated by reference). It has one of the lowest friction coefficients when paired with metal alloys. The present 3-phase manufacturing process of polymer synthesis in powder form, granulation in the form of beads, and extrusion into rods or sheets, followed by machining into the required shape, leads, however, to very high costs. PEEK has therefore only been used sparingly in the past.
What is needed is a method of bypassing the costly process of manufacturing for example PEEK implants from bulk material. There is furthermore a need to create implants with a mechanical structure close to that of natural bone. The invention described below solves many of the problems in the art.